The broad objective of this research proposal is to gain insight into how axon guidance is controlled at the molecular level. This will be completed through x-ray crystallography studies of both a receptor (Roundabout), and a ligand (Slit), which has been demonstrated to function as both a positive and negative regulator, of axon guidance. The specific aims are: l) Expression, purification and crystallization of cytoplasmic and Ig domains of the Roundabout (Robo) receptor. 2) Structure determination of those domains of Robo that are crystallized. 3) Determination of the structure of the Ig domains of Robo in complex with the minimal Slit binding region. Our current understanding of the mechanism of Slit/Robo function is that Slit binds to the ectodomain of Robo and translates a response to the cytoplasmic region where negative response, elements are activated. How Robo activates these response elements is the specific objective of this research proposal. Determining the structure of the cytoplasmic domain of Robo will provide an understanding as to the overall three-dimensional fold of this domain and aid in understanding how Robo functions in a negative manner. Completion of all the above aims will provide, for the first time, a working model of axon guidance ligand/receptor function at the molecular level. This will allow for additional research to be completed and further our understanding of axon guidance cues and receptors to aid in the advancement of nerve regeneration.